You can read up more about it at
http://emdrive.com/
The site has a paper published with the theory, however since there is no peer review and all the math is way beyond me, I'd have to assume there are holes in it.
There have been multiple tests done, not just by NASA. The most significant was by a Chinese research team which produced 720mN thrust based on a 2500W input.
The latest paper by the inventor goes on about how we could to a mission to the nearest star 4 light years away in just 10 years, reaching a speed of up to 0.67c.
It's been 14 years since the inventor demonstrated this technology. The fact that NASA is exploring it is interesting, however the fact that it hasn't taken the world by storm is more telling IMO